A. Field of the Invention
The present invention relates to apparatus for use in oil well drilling operations. More particularly, the invention relates to mechanisms for preventing pressurized subsurface liquids or gases from blowing out and upwards through a well hole.
B. Discussion of Background Art
In drilling for natural gas or liquid petroleum, a drill string consisting of many lengths of threaded pipes screwed together and terminated by a drill bit head is used to bore through rock and soil. The drill bit head has a larger diameter than the pipes forming the drill string above it. The upper end of the drill string is rotated to transmit a rotary boring action to the drill bit head.
A specially formulated mud is introduced into an opening in an upper drill pipe, flowing downward through the hollow interior of the pipes in the drill string and out through small holes or jets in the drill bit head. Since the drill bit head has a larger diameter than the drill string above it, an elongated annular space is created during the drilling process which permits the mud to flow upwards to the surface. The purpose of the mud is to lubricate the rotating drill string, and to provide a downward hydrostatic pressure which counteracts pressure which might be encountered in subsurface gas pockets.
In normal oil well drilling operations, it is not uncommon to encounter subsurface gas pockets whose pressure is much greater than could be resisted by the hydrostatic pressure of the elongated annular column of drilling mud. To prevent the explosive and potentially dangerous and expensive release as gas and/or liquid under pressure upwards out through the drilling hole, blowout preventers are used. Blowout preventers are mounted in a pipe casing surrounding a drill hole, near the upper end of the hole.
Typical blowout preventers have a resilient sealing means which can be caused to tightly grip the outer circumferential surfaces of various diameter drill string components, preventing pressure from subterranean gas pockets from blowing out material along the drill string. Usually, the resilient sealing means of a blowout preventer is so designed as to permit abutting contact of a plurality of individual sealing segments, when all elements of a drill string are removed from the casing. This permits complete shutoff of the well, even with all drill string elements removed. Most oil well blowout preventers are remotely actuatable, as by a hydraulic pressure source near the drill hole opening having pressure lines running down to the blowout preventer.
Blowout preventers having resilient sealing means are disclosed in U.S Pat. No. 3,323,773, R. W. Walker, June 6, 1967, and U.S. Pat. No. 3,667,721, issued June 6, 1972 to A. N. Vujasinovic.
Prior blowout preventers, including those disclosed in the above-identified U.S. patents, typically use a circularly spaced array of curved metal segments which are contained slidably in a hemispherical cavity and pushed upwards by a hydraulic piston to effect a reduction in diameter of an upward entrance bore to the spherical cavity, through which drill string components are inserted. The curved metal segments are held in a circumferentially spaced relationship by being molded integrally into a resilient rubber matrix having a generally cylindrical interior shape. When the sealing element comprising the curved metal segments and resilient matrix are moved upwards, the inner cylindrical rubber surface is forced to cold flow inwards towards the outer circumferential surface of the drill string components within the blowout preventer, thereby effecting a seal and preventing pressurized fluids below the blowout preventer from escaping upwards. In some prior art blowout preventers, sufficient movement of rubber inwards is afforded to completely seal the bore through the blowout preventer, even with all drill string components withdrawn.
Existing blowout preventers can damage drill string components under certain conditions. Since the metal segments used in the sealing element of some blowout preventers are non-resiliently translated upwards and inwards by the actuator piston, the upper inner edges of the segments can contact the circumferential surface of drill string components with radial compressive forces sufficient to damage the component. With this and other limitations of existing blowout preventers in mind, the present invention was conceived of.